The present invention relates to a CVD batch loading method used in connection with coating of cutting inserts suitable for a rational production in larger scale allowing fully automatic CVD loading.
Chemical Vapor Deposition (CVD) of wear resistant coatings on cemented carbide inserts has been an industrial practice for more than 20 years. The CVD technique has extensively been discussed in the literature.
In order to minimize the production costs for coated inserts, large sized (10,000-15,000 inserts) CVD batches are generally used. Rational loading and unloading of the inserts from the CVD reactor is thus required. Transportation of the whole load or parts of the load must be possible to be carded out safely, particularly prior to the coating operation.
Another problem related to mass volume CVD is to find an appropriate loading arrangement of the inserts that will allow a well-controlled gas flow through the CVD reactor in order to ensure high and even coating quality throughout the whole load.
Yet another problem is to avoid contact marks on the inserts formed during the coating process in the zone between the inserts and their support. Contact marks on the inserts are not acceptable from a cosmetic point of view and also since they may deteriorate the cutting performance of the coated inserts.
There are many different batch loading systems or methods known in the art.
In one loading arrangement, the inserts are allowed to hang in slits in a tray. This method has the disadvantage of leaving contact marks on the clearance faces of the inserts. The system is unstable and there is always a potential risk during transportation that the inserts fall over and completely rearrange the packing. In addition, all surfaces of an insert with this type of loading will not be exposed to the same amount of gas flow during the coating process. This may result in serious variation in the coating thickness of any particular insert and hence the quality of the coating.
In another similar system, the inserts hang between rods. This system suffers from the same drawbacks as those mentioned above.
In the so-called net loading arrangement, the inserts are placed on woven metal nets. The load is built up by piling each metal net on top of each other. When using this method: a) the gas supply to the regions between the inserts and the metal net wire may be insufficient; b) care has to be taken during transportation to avoid that inserts slide together resulting in contact marks between individual inserts; and c) contact marks between the nets and the inserts are always formed.
In yet another method, the inserts are threaded up on a rod. Spacers between each insert are generally used to avoid contact between the inserts. The rods may be oriented horizontally or vertically in the CVD reactor. The main drawbacks of this method are the very complicated and time consuming loading and unloading procedures. Contact marks between the spacer and the insert are difficult to avoid. This method can be applied to inserts having a central hole for clamping.
In EP 454,686, a loading system is disclosed in which the inserts are stacked on top of each other on a central pin, possibly with a spacer between. As a result, the inserts will be only partially coated. This method is aimed for plasma CVD and will not be fully applicable to ordinary CVD since the inserts will be "glued" or grown together during an ordinary CVD process.